I have managed to get a result so I thought I should post the answer.
Firstly I must thank Rob Davies who investigated the British Origins site for me. He had no luck either and its nice to know the problem is not just my incompetence!
Having repaired permissions and re-started I thought I should have one more try with Internet Explorer as that worked before.
But no luck. When I looked at the helpers Graphic Converter was listed as the application but ViewTIFF was listed as the download handler again (I had changed it to GC which was what worked previously). So I chucked ViewTiff out of the plugins and decided to have one more try using their latest recommended TIFF viewer XnView. I could not set it as the helper application because of its lack of a file creator code so I left GC as the helper with XnView as the download handler. It worked!!!
The downloaded file has the extension ASPX though the GC icon says 'TIFF'. I wondered if this was why it would not open in Preview and tried changing it to TIF and TIFF but that did not work. This does not matter too much as the images need the brightness and contrast tweaked a bit, so I adjust then save in GC and the resulting file will open in Preview.
Not as good as being able to view before downloading but better than nothing. Eventually Ancestry will get the 1841 and 1851 censuses and I shall abandon British Origins!
The good news is that XnView is free, I would have had to pay for ViewTIFF in 15 days. The bad news is that I don't know how to use XnView in Safari. I shall definitely try FireFox and see if it will work with that, I don't like IE!
Thanks again to all the helpful WAMUG folk and best wishes from Diana
